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This study aimed to investigate the internal dislocation law of coal seam working face under coal pillars with different widths and evaluate the safety of using the combined mining of close-distance coal seams under different layer spacing. Based on elastic–plastic mechanics and mine pressure theory, theoretical calculation and numerical simulation were implemented to establish a numerical model consistent with the actual production. Based on the theoretical and simulation results, the abutment stress distribution of surrounding rock in coal seam stope was analyzed and the stress transfer law of coal pillar floor under the influence of different coal pillar width and floor depth was studied. Finally, this paper analyzed the limit span of goaf under two different beam structures. The results indicate that with the same coal seam distance, the interior dislocation distance increases linearly with the increase of coal pillar width. With the increase of floor depth, the vertical stress under the coal pillar decreases, but the influence area increases slightly. Finally, the ultimate spans of two different beam structures were calculated. In summary, this paper explored the strata behavior, i.e., the deformation and failure mechanism of surrounding rock in coal seam mining. The research results demonstrated the feasibility of using the combined mining method in close-distance coal seams. The methodology proposed in this paper could be implied in similar working environments, and this study provides new insights into the mining technology in lower coal seams. [ABSTRACT FROM AUTHOR]

This paper aims at to establish a novel approach of gravity blind backfilling method for abandoned mine voids using suitable backfill materials. Gravity blind backfilling is an important aspect for controlled movement of the overlying strata and efficient reduction of surface subsidence over unapproachable water-logged as well as dry underground coal mines. In this investigation, studies have been carried out in a fabricated transparent scaled mine model of a section of the bord and pillar underground mine using gravity blind backfilling method. The objectives of the paper is to determine the appropriate method with suitable backfill material through laboratory trial studies to develop an empirical equation among void volume, solid flowrate and solid concentration in gravity blind backfilling method. Since sudden jamming is a major hurdle in filling-up of a large scale areas from one borehole, an attempt has also been made to develop an electric based alarm indication parameter. Empirical relationships have been developed for estimation of maximum filled-up void area covered by backfilled solids after filling. Practical observation have been recorded about shape of filled up area, filled up height and directional spread of slurry in gravity blind backfilling. Electric sensor-based alarm system can predict water levels at different head and indicate the arrival of final stage of filling after which not more than 25% of filling will be possible with the same flowrate and concentration. The best (i.e. bottom ash 30% and sand 70%) and second-best solid composition (i.e. bottom ash 40% and sand 60%) of bottom ash and sand for blind backfilling purpose are determined on the basis of the localized spread of backfill solids without any sign of sliding towards the dip direction, full compaction up to roof of the gallery and greater load-bearing capacity. The coefficient of correlation came to be 0.81 which showed the different parameters has a strong inter relationships amongst each other. [ABSTRACT FROM AUTHOR]

Chimney subsidence can cause surface subsidence (ground settlement) in the form of a pit or sag when there is a mine roof collapse in a mine entry. Surface subsidence occurs when the roof collapse propagates upward and does not choke itself off due to the associated volume expansion of the rubblized fallen materials and where no sufficiently competent rock is present to bridge the cavity. This paper contains a review and evaluation of reported case history data associated with occurrences of surface subsidence related to chimney subsidence. This analysis included investigating the chimney subsidence height potential including relative to the overburden rock conditions, extraction height, mine depth, the width of entry and the roof rock fall distance. Also, this paper provides a summary of the results of an assessment of the bridging capacity of various more common competent coal measures in the roof. This numerical analysis was performed considering linear arch conditions. Parameters which were investigated in this analysis included the competent roof rock strength, thickness, depth, and spanning distance. [ABSTRACT FROM AUTHOR]

The primary key stratum (PKS) is an important research object in ground control, and all strata up to the surface are controlled by it. However, the geological conditions of coal mining are complex, and coal mining under different geological conditions will cause different degrees of impact on PKS. This paper takes the Meihuajing coal mine as an example, theoretical analysis, numerical simulation and geological condition analysis are comprehensively applied to study the subsidence mechanisms of PKS under the influence of different factors. The results show that subsidence of the PKS is directly proportional to the coal thickness (M) and buried depth (D), and inversely proportional to the distance between the coal seam and the primary key stratum (H). The order of the disturbance degree with each factor on the maximum subsidence of the PKS is M > D > H. It is demonstrated that the disturbance impact of coal seam mining on the PKS in an area with thick coal seams needs to be paid more attention. The research results guided adjusting and reducing the disturbance impact of mining on the PKS. [ABSTRACT FROM AUTHOR]

In recent years, with the extension of the coal mine at depth, rock bursts are more and more frequent. Therefore, the research of roadway support and pressure relief has become the key to prevent rock burst. In this paper, the primary high strength support and hydraulic fracturing pressure relief measures of roadway are studied and analysed. The primary high strength support emphasizes the matching characteristics of support materials, strengthen the protection of the active support to the roadway, and realize an integrated support system. Considering strong burst-prone area, the hydraulic fracturing is used to achieve pressure relief of surrounding rock mass, which can release and transfer the energy within the range of pressure relief. When there is energy release outside the range of protection system, the pressure relief region can absorb the energy so as to achieve the purpose of protecting the roadway. A field engineering case is applicated, the results shown that the stress of roadway surrounding rock and micro seismic energy significantly decreases, reducing the possibility of rock bursts. [ABSTRACT FROM AUTHOR]

Historically, the idea of designing and installing rockbolts in rockmasses can be traced back to the construction of the Snowy Mountain hydroelectric scheme in Australia, which was completed in 1974. The use of rockbolts as a ground support system has been widely applied to enhance the stability of structures in mining and to resist the movement of keyblocks in underground excavations. The strength of a rockbolt as a rock support tool lies in its ability to improve the tensile and shear strength needed to maintain the load-bearing capacity of a rockmass. Currently, rockbolts are extensively used as reinforcement methods in mining and civil engineering projects to support deforming rockmasses in coal and hard rock mining, tunnelling and underground openings. The performance of a rockbolt depends on the design parameters such as load-bearing capacity of the bolt, strength of the bolt, bolt diameter, length of the bolt and bolt spacing. This study provides a detailed review of the various applications of rockbolts in mining operations. In addition, the integration of smart sensors in rockbolts to enhance monitoring of displacement measurements, stresses, subsidence of pillars and ground pressure on structures are discussed in this paper. The use of smart rockbolt devices, which has improved the monitoring and evaluation of critical parameters in rockbolts such as the axial force, corrosion occurrence, grout quality and resin delamination, are also discussed. This review shows that with the numerous applications of rockbolts in mining, especially in underground excavations and tunneling engineering, rockbolts can be used as supports in rocks with different geological conditions. Nevertheless, this paper highlights that there are still some areas that need improvement in rockbolting technology. [ABSTRACT FROM AUTHOR]

Backfilling in goaf after pillar extraction seems to be a promising technology to help exploit these standing pillars. This paper presents the assessment of the impact of sand-stowing-based backfill on the peak strength and post-failure behavior of coal pillars. Rectangular coal samples were prepared and tested with different proportions of sand stowing, and a 3D finite-difference code was used for the numerical simulation to achieve the objective of the study. In laboratory experiments, the proportion of sand stowing was set at 0%, 60%, 70%, 80%, 90%, and 100% and tested in a servo-controlled machine, whereas, in numerical simulation, the percentage of sand stowing varied from 9 to 91% along with the coal pillar width-to-height ratios ranging from 1.5 to 5. The experimental and numerical results demonstrated that the average pillar strength increased as the proportion of stowing increased. The overall strength increased by 22% in the experimental study and about 28% to 32% in numerical simulation. The load vs. displacement characteristic changed from brittle to ductile as the sand proportion increases and the width-to-height ratio increased. [ABSTRACT FROM AUTHOR]

Primary and secondary hole defects in the process of coal mining will have an important impact on the stability of coal, therefore, the study of the mechanical and related characteristics of coal containing hole defects is of great significance for safe production and engineering practice. In this paper, the RMT-150 rock mechanics system is used to carry out uniaxial compression damage tests on coal with different hole sizes, and the whole process is recorded by simultaneous infrared radiation monitoring, based on which the influence of hole size on the change of physical properties and infrared radiation temperature response is analyzed. The results found that: the hole defect has a deteriorating effect on the strength of the coal, and the hole size is negatively correlated with the uniaxial compressive strength and elastic modulus. When the coal doesn't contain a hole or the hole size is small, the load damage is mainly shear, and as the hole size increases, the tensile damage characteristics gradually become significant. Hole defects cause the initial differential maximum infrared radiation temperature and the initial differential lowest infrared radiation temperature of coal surface thermal images to diverge in a " < " trend, while the initial differential average infrared radiation temperature monotonicity is obvious. The slope of linear change in the middle section of the coefficient of variation of infrared radiation temperature gradually increases with the increase of the hole, while the change of entropy value is generally higher than that of intact coal. The infrared temperature field of the coal containing hole is approximately Bi-gaussian distribution, and the instantaneous trend of the fitted distribution curve shifts to the right and overall to the right as the test progresses, and the curve shifts from a lean and tall form to a short and fat form. [ABSTRACT FROM AUTHOR]

The severe deformation of surrounding rock and the easy failure of the anchoring structure of roadway under large dynamic pressure influence in the short distance are the urgent issues to be paid more attention. Based on this problem, this paper is a case study on the instability mechanism and control technology of surrounding rock in the deep roadway under dynamic mining loading in the Gubei Coal Mine of China. The vertical distance from the − 575 m roadway of 11–2 mining area and mining working face is only 68 m. Firstly, the main factors influencing the surrounding rock's severe deformation are determined by the analysis of site investigation, surrounding rock composition and structure, overlying strata failure measurement, and borehole camera detection. The stress distribution, displacement distribution, plastic zone, and lining stress distribution of roadway of three reinforcement schemes, including nine working conditions, were calculated by numerical simulation for ordinary bolt support, bolt-support + grouting of roof and two sides, and bolt-support + full section grouting. Then the combined support technology system of "36 U-shaped steel + bolt + anchor cable + grouting" was proposed. The field monitoring results showed that the deformation of two sides and roof were only 20 mm and 16 mm, respectively, during the 65 day monitoring period. Furthermore, the cracks in the surrounding rock are almost filled with grouting materials, and the overall stability of roadway surrounding rock was completely achieved. This successful case study demonstrated that the optimized supporting structure is reasonable and the deformation of surrounding rock has been effectively controlled. [ABSTRACT FROM AUTHOR]

Longwall mining operation is mainly accompanied by the roof fall phenomenon and out of seam dilution (OSD). These occurrences are strongly dependent on the quality of the face's roof and floor. Therefore, the existence of discontinuities in the floor and roof reduces their quality, increasing the intensity of roof fall and OSD. This paper suggests a synthetic rock mass (SRM) model which is able to simulate roof behavior by initiating and tracking of fractures in cave mining, in order to predict the roof fall and OSD and to evaluate their event risk. For this purpose, longwall mining, including excavation of coal, installation of powered support, and caving the roof, has been successfully established in this numerical model using the PFC2D approach. The risk analysis in a panel of Tabas mine using the matrix risk model indicated that the risk level of roof fall is moderate and the risk level of OSD is high. The results of SRM modeling are consistent with previous studies and field observations at the Tabas coal mine, indicating that the SRM model is a powerful technique for studying the behavior of discontinuities in mining. [ABSTRACT FROM AUTHOR]

Coal mining is threatened by the aquifers below coal seams, and grouting is used to reconstruct the aquifers before mining. However, water gushing-out sometimes still occurs from the grouted aquifers. In this paper, two aspects of grouting in a vertical fracture are studied by numerical methods, including the slurry diffusion distance and the influence of grouting stone body on water inflow and water pressure. First, a grouting model consisting of a fracture and a borehole is proposed based on the hydrogeological conditions and grouting processes. Two hydrodynamic models controlled by the grouting stone body are established to study the variation of the water inflow and the water pressure in the filled section of the fracture. Then, the numerical calculation is carried out in detail. The numerical results show that the diffusion distance following the flow direction is significantly greater than the distance against the flow direction. The dynamic viscosity is successfully used to simulate the plugging effect of the grouting stone body. For a same fracture section, when the ratio of the maximum filled area to the fracture section area before grouting is less than 90%, the flow velocity presents an upward trend with the flow section decreasing due to the Venturi effect caused by the grouting stone. When the ratio of these two areas is more than 90%, the change of the flow velocity can be explained by the equal percentage characteristic of the valve. The data monitored by typical boreholes during grouting show that the water inflow and the water pressure both reduce after grouting or water inflow decrease and the water pressure keep unchanged after grouting, which is approximately in line with the fracture flow pattern controlled by the valve open degree. This study is conducive to the theoretical research of grouting engineering. [ABSTRACT FROM AUTHOR]

To study the influence of changes in the coal seam mechanical characteristics at depth on the stress relief effect of a borehole, this paper established the relationships between the borehole spacing, elastic modulus, and Poisson's ratio of the coal seam. It then analyzed the stress relief effect of different borehole spacings in different coal qualities in a deep coal seam, using the FLAC3D numerical simulation software. The results indicated that after boreholes with different spacings were drilled in the deep coal seam, the peak stress value of the roadway-surrounding rock was transferred to the deep coal body, and the original peak stress position was reduced to varying degrees. Under the premise of ensuring the stress relief effect and improving stress relief efficiency, the relationship between borehole spacing and the elastic modulus of the coal seam was a negative exponential function, whereas the spacing of boreholes and Poisson's ratio of the coal seam had a logarithmic function relationship. The research conclusions were applied to the − 1000 West No. 1 mining area in the Tangkou coal mine, which achieved good stress relief results. [ABSTRACT FROM AUTHOR]

Mine tremors in coal mines have evolved into an important factor affecting the safety of underground production and surface buildings. In order to address the problem of frequent mine tremors in Dongtan coal mine, a model of repetitive mine tremors in multi-layered key layers was established by using theoretical analysis and field observation. The process of mining tremor energy accumulation and release was analyzed, and the theoretical equation of mining tremor energy calculation was proposed, and the mechanism of occurrence of repetitive mine seismic was obtained. The results show that the concentrated release of gravitational potential energy and elastic strain energy due to the periodic fracture of multiple key layers is the fundamental cause for the frequent repetitive mine tremors in Dongtan coal mine. The energy of repeated mine tremors in multi-layer key stratum mainly comes from two parts, one is the release of gravitational potential energy caused by the key-layer fracture, and the other is the elastic strain energy released by the fracture of the key-layer and its controlling rock layer. The paper successfully predicted the maximum mine tremor magnitude of 3.15 Richter scale during the mining process of 6305 working face, which can correspond well with the maximum mine tremor magnitude of 3.04 Richter scale monitored by microseismic equipment. The research results can provide a reference for predicting the maximum magnitude of mine tremors under similar stratigraphic conditions. [ABSTRACT FROM AUTHOR]

The stability of remnant coal pillars is of great significance for safe mining of coal resources around them. The remnant coal pillars are often soaked in mine water with a certain height. Different water levels (or depths) may affect the stability of remnant coal pillar differently. In this paper, uniaxial compression tests were carried out on coal specimens with different water soaked depths. Deformation and damage characteristics of coal specimens under loading were analyzed through acoustic emission (AE) and digital image correlation techniques. The results show that the AE energy curve of each partially soaked coal specimens presents double peak, while that of fully soaked coal specimen has only single peak. As the soaked depth increases, the AE b value gradually decreases, which indicates that the micro-fracture of coal specimens decrease with the increase of water soaked depth. During deformation and damage, partially soaked coal specimens have obvious localized strain, and the local strain concentration area is basically distributed in the dry–wet transition area. While the surface of the fully soaked coal specimen presents relatively uniform deformation. Partially distributed water within coal matrix increases the heterogeneity of the coal specimens. These results reveal the deformation and damage characteristics of coal with different water soaked depths, and provide a useful reference for reasonably evaluating the stability of coal pillars. [ABSTRACT FROM AUTHOR]

The gob-side retaining can save coal pillar of mining area and improve coal recovery rate of working face. Meanwhile, this technology helps relieve the tension of coal mining, and reduce the problem of gas accumulation in the tracking corner of working face. Therefore, the gob-side entry retaining ranks in the main development goals of efficient coal mining. The thickness, dip and buried depth of coal seam in the mentioned 3304 working face are large, so it is difficult to control the surrounding rock of the retaining roadway. This paper mainly studies the safe and reliable supporting parameters of the gob-side entry retaining in the 3304 mining area. Firstly, the width and strength of roadway wall filling body and bolt support scheme were designed by orthogonal test method. Then, numerical simulation software is used to comprehensively analyze the influence of the strength and width of roadside support body on the surrounding rock pressure. And the deformation law of surrounding rock under different reinforcing support parameters is further analyzed. Finally, the reasonable support is determined. The width of roadway side filling body is 1.5 m; The strength of the concrete used is C25; The practice proves that the roadway retention scheme and support parameter adopted are effective, which have reference value for similar projects. [ABSTRACT FROM AUTHOR]

Gas is one of the main factors causing coal mine accidents in China. Understanding the mechanism of the influence of mine pressure on coal seam gas emission has important theoretical guiding significance for coal mine enterprises to be able to control gas. This paper chose the 72,909 working face in the Du'erping coal mine as the engineering background, analyzed the characteristics of the mine pressure in the working face and the driving face through numerical simulation, and found that the power of the gas emission in the working face is the mine pressure, while that of gas emission in the driving face is the gas pressure gradient. Based on the field monitoring data of the hydraulic support in the working face and the single pillar in the mining roadway, the law of mine pressure was obtained. Combined with the monitoring data of the number of drill cuttings and gas in different positions of the mining roadway and the gas concentration change in the upper corner of the working face during mining, the influence of the advanced support pressure and periodic pressure on the gas emission of the coal seam was analyzed. The results show that the advanced support pressure affects the coal seam gas emission by affecting the coal seam permeability coefficient. With the increase in the support pressure, the coal seam permeability coefficient decreases, and the coal seam gas emission decreases. During the period of periodic pressure, the advanced support pressure moves forward, forming a pressure relief zone in front of the working face, which is conducive to the emission of coal seam gas, and periodic pressure provides power for the gas emission. [ABSTRACT FROM AUTHOR]

The slow excavation speed of coal roadways has always been a key factor restricting the safe and efficient production of large-scale coal mines in China, and the problem of unbalanced mining replacement caused by this is widespread. This paper takes the S1231 heading face of the Ningtiaota coal mine of Shaanxi Coal and Chemical Industry Group as the research object, analyzes the characteristics of the stress evolution of coal roadway driving, reveals the principle of low-density cross-border support, and proposes a low-density cross-border support plan. Using FLAC3D to study the roadway stress-displacement evolution law of the new support scheme during the driving and mining phases, the results show that the peak stress during the driving is 5.3 MPa, and the coal pillar side stress concentration is the most obvious during the mining period, with the peak value being 7.9 MPa. The moving distances of the two banks are both 10 mm, which verifies the feasibility of low-density cross-border support. Field application shows that during roadway excavation, the amount of roof subsidence and the displacement of the two sides are 9 mm and 11 mm, respectively, and the development depth of roof cracks is controlled within 0.5 m. The overall control effect is good, and the speed of coal roadway driving is increased by 77.19% compared with the original. The new support builds a thick roof anchor structure to ensure the safety and stability of the roadway. At the same time, by reducing the number of bolts, the bolt support time has been greatly reduced, effectively alleviating the tight situation of mining replacement, and providing solutions for mines under the same conditions. [ABSTRACT FROM AUTHOR]

The No. 3 coal seam is one of the primary mineable coal seams in Du'erping coal mine of Shanxi Coking Coal Group Co., Ltd. The direct roof of No. 3 coal seam is a compound roof with two layers of 0.3–0.5 m extremely soft coal seams in the unstable rock stratum. The mining roadway has a large amount of deformation and is difficult to support. This paper takes 63,409 ventilation roadway of Du'erping coal mine as the engineering background. The RFPA2D numerical simulation software was used to compare and analyze the distribution characteristics of surrounding rock fracture zone and surrounding rock deformation in the mining roadway of the general roof and compound roof under the conditions of coal body–coal body and coal body–coal pillar. The supporting form and main supporting parameters of the mining roadway with compound roof were determined. Through simulation and comparison, the deformation of the mining roadway with compound roof under unsupported and three different support conditions were analyzed, and the combined support method of "bolt + anchor cable + metal mesh" was field tested. By analyzing the monitoring data of the roadway roof subsidence, floor heave, ribs displacement and roof abscission layer, the results show that the combined support method of "bolt + anchor cable + metal mesh" effectively controls the large deformation of the mining roadway with compound roof, improves the stability of the surrounding rock, suppresses the occurrence of roof fall and rib spalling, and ensure the safety of the roadway. [ABSTRACT FROM AUTHOR]

Based on the S1201-2 large height mining in the 2–2 coal seam of Ningtiaota colliery with on-site microseismic measurement, physical simulation and theoretical analysis methods, this paper explores the rule of roof movement in thick coal seams with roof cutting and non-pillar (hereinafter referred to as RCN-P) mining, so as to obtain scientific and effective theoretical basis for entry support and to summarize the regional structural characteristics and dynamic periodic fracture characteristics. As can be seen from microseismic events, the entry roof is featured by "two zones and one line" along the horizontal direction, namely, the crack generation area, the roof movement area. Additionally, and the obvious lateral breaking of the entry roof on the coal wall is a typical feature of the thick coal seam with RCN-P mining. The roof is vertically divided into "three zones", the crack generation area, the roof movement area and the crack development area. The roof cutting activity mainly affects the overburden activity within the basic roof height range, which is also the roof movement area. In addition, the distribution frequency and the intensity of microseismic events indicate the roof periodic breaking characteristics. The "breaking pressure relief," "advanced crack development," and "the limit breaking state" of roof breaking corresponds to the initial, middle, and final stage of breaking in the periodic weighting process, respectively. Compared with the normal mining, the RCN-P mining reduces the periodic weighting length and increases the pressure strength. As is shown in the physical simulation experiment, the basic roof and the cutting control layer in the "regional structural characteristics" constitute the "large" and "small" structures with RCN-P mining. The basic roof key layer is the core to control the stability of the strata, and the breaking process from the cantilever beam to the short masonry beam of the roof-cutting control layer is the main cause of the entry stress. Correspondingly, the basic structure model of "short masonry-hinged" roof was proposed and the calculation method of support was established for the entry with RCN-P mining in thick coal seam, providing a research foundation for scientific and effective rock formation control. [ABSTRACT FROM AUTHOR]

To ensure the safety of coal mining, lots of coal pillars were reserved in the coal mine. The stability of coal pillar and roof composite structures determines the safety of stope. Under the action of cyclic loading and unloading such as mining face, chamber blasting, and roadway excavation, the coal pillar, and roof composite structure will gradually instability, which will affect the safety production of the coal mine. Therefore, this paper takes the dynamic disaster of coal pillar and roof composite structure as the research background, carries on the cyclic loading and unloading test of the coal-rock composite structure with different coal-rock height ratios, and studies the influence of coal-rock height ratio on the progressive instability characteristics, mechanical properties and damage variables of the coal-rock composite structure. The crack initiation stress, crack propagation time and failure pattern of coal-rock composite structure are analyzed. The improved load-unload response ratio considering the damage variable is established, and the stability of the coal-rock composite structure under cyclic loading and unloading is quantitatively analyzed. It is concluded that the larger the coal-rock height ratio is, the greater the load-unload response ratio of the composite structure is, and the more unstable the composite structure is. This conclusion is consistent with the crack initiation stress and fracture propagation time, and can quantitatively reflect the progressive instability process of coal-rock composite structure, and provide a basis for revealing the dynamic disaster of coal pillar and roof composite structure. [ABSTRACT FROM AUTHOR]

With the increasing tension of current coal resources and the increasing depth of coal mining in China, Non Coal Pillar Gob-side Entry Retaining Technology has become a preferred coal mining method in underground coal mines. In the current study of the non coal pillar gob-side entry retaining mining with roof cutting and pressure releasing, the theory and design of roof cutting under near-level conditions have formed a certain system. However, there is a lack of systematic research on the design of the roof cutting and the connection requirement of the cutting seam and other aspects about the inclined coal seam. Therefore, choosing the concrete representative target mines as the research foundation, using mechanics analysis, geometric analysis, numerical simulation and other means, on the basis of considering the change of mining height, this paper mainly studies about the effect of coal seam inclination on roof slit depth and angle, then get the three steps of the cutting height design, the three considerations of the roof cutting angle design and the connection rate requirement of the roof cutting. At last the key roof cutting parameters of the three target mines are calculated accordingly. Based on the existing research, this paper puts forward the concept and design method of roof cutting connectivity rate. The design system of roof cutting seam parameters of non coal pillar gob-side entry retaining mining with roof cutting and pressure releasing is further improved, and the corresponding calculation method is deduced for the design of the key parameters of the inclined coal seam and the gently inclined coal seam, and proposing the second roof cutting scheme of the upper side gob-side entry retaining. [ABSTRACT FROM AUTHOR]

In order to study the response characteristics of reflected trough in-seam wave in fault bearing coal seam, find out the geological structure of coal mine, and ensure the scientific and safe mining of coal mine, the physical property parameters of coal and rock samples of No.3 Coal Seam in Shuangliu Coal Mine were determined, and the three-dimensional numerical analysis model of homogeneous isotropic medium was established. The full wave field forward modeling of elastic wave equation was carried out by using the finite difference method. In this paper, the dispersion curve of No.3 Coal Seam in-seam wave detection in Shuangliu Coal Mine is studied. The results show that under the geological conditions of Shuangliu Coal Mine, the dispersion curves of in-seam wave are obviously different when the coal thickness is 1 m, 3 m, 5 m and 8 m; The dispersion curve of reflected in-seam wave in goaf is disordered, which is obviously different from that of normal coal; The Airy phase velocity of in-seam wave in No.3 coal seam of Shuangliu Coal Mine is about 100 0 m/s, and the dominant frequency is 140 Hz. The conclusions are as follows: the formation of in-seam wave in the mining area is related to the direct roof and floor, but not to the indirect roof and floor; when the wave velocity difference between the roof and floor is large, the better dispersion response of Airy phase. [ABSTRACT FROM AUTHOR]

This paper presents a mining technology that enables the continuous conversion from top coal caving (TCC) to multi-slice mining (MSM). The technology helps operations overcome challenges with mining thick bifurcated coal seams, particularly the challenge of rock-parting seams affecting the caving of TCC mining methods. Findings show that the relationship between the rock-parting thickness and the caveability of top coal can be divided into 3 phases, including a complete caving phase, a transition phase, and a stable phase. This continuous conversion technology was applied in the Zhongwei Coal Mine in which TCC was used to extract the initial 260 m of the study area where the rock-parting seam was relatively thin before a transition to MSM was made due to the rock-parting seam becoming significantly thicker. The continuous conversion to MSM involved the working face advancing upward at an angle of 10° until the rock-parting seam could serve as the immediate floor for the upper slice of the coal seam. The rock-parting seam floor also acted as the immediate roof for the working face of the subsequently mined lower slice. The application of this continuous conversion mining technology obtained good application results. [ABSTRACT FROM AUTHOR]

Airtight blast door, as the critical component of underground coal mine, requires high quality. Topology and shape optimization were seldom applied in the structure design of products though they were proposed early. Based on those optimization methods and the experience of structure design, a new kind of airtight blast door was developed in this paper. The main component weight of thus kind of door is 14 % smaller than the current airtight blast door without reducing the blast and impact behavior. The typical optimization method and the example presented in this paper are useful for the original development and optimization design in other products. [ABSTRACT FROM AUTHOR]

Ground water inrush from underlying coal seam aquifers is a serious geohazard during coal mining in China. In order to effectively predict coal floor water inrush, the evaluation index system and evaluation standard for coal floor water inrush, containing quantitative indexes and qualitative indexes, is established on the base of conditions of aquifer and aquiclude, geologic structure and mining disturbance. Simultaneously, grey relational analysis and analytic hierarchy process are used to establish an evaluation model which efficiently overcomes the uncertainty between indexes of water inrush effects and really reflects the degree of importance for each index of water inrush effects. The model is applied to the typical working face of Yuzhou coalfield in the north of China to demonstrate the evaluation process. Compared to the water inrush coefficient method which is widely used for evaluating coal floor water inrush, the presented evaluation model in this paper accords more with the susceptibility of coal floor water inrush by multi-factor and is of nonlinear dynamic characteristics of highly complicated formation mechanism. This method offers a new tool in the assessment of ground water inrush in mine. [ABSTRACT FROM AUTHOR]

Based on the 88,302 working face of Baode coal mine, in order to provide the basis for the auxiliary roadway maintenance of the working face, this paper introduces the test principle, test method, test process and result analysis of the loose circle. The analysis shows that the loose range of the two ribs of the auxiliary haulage roadway in 88,302 working face increases gradually with the reduction of the distance from the working face. Affected by mining, the maximum loosening range of roadway coal pillar side is 2.4 m, and the maximum loosening range of coal wall side is 2.2 m. It provides the basis for determining the reasonable bolt support parameters, and provides the practice for the loose circle test of the roadway under similar conditions. [ABSTRACT FROM AUTHOR]

The dip angle of coal seam in Guanyinshan coal mine is between 32° and 39° and the first mining area is X11 and X12. According to the caving degree of the immediate roof in the gob, the intensity of pressure of the main roof is analyzed, and the dynamic load coefficient of the main roof is small. The three-dimensional simulation software FLAC is used to calculate the distance of initial caving and distance of initial pressure of working face. The initial caving distance of the immediate roof at each part of the working face is about 4 m, and the initial pressure distance of main roof is about 20 m on average. This paper analyzes the caving of the main roof: when the distance of initial pressure is 20 m, the caving height of the main roof is about 4 m; when it is pushed to 30 m, the caving height of the main roof is about 10 m; with the continuous mining of the working face, the caving height of the main roof is also increasing, when it is pushed to 90 m, the caving height of the main roof reaches 18 m. Through the analysis of the regular of underground pressure in large dip angle soft roof and floor coal seam mining, it can guide the production. [ABSTRACT FROM AUTHOR]

Accurate prediction of surface deformation due to the extraction of underground coal seams is a significant challenge in geotechnical engineering. This task is further compounded by the growing trend for coal to be extracted under buildings by leaving coal pillars in place. To rationalize the design of mining schemes to ensure effective control of surface deformation, precise 3D numerical models of different mining schemes were developed in this paper. Two schemes were compared and analyzed based on the results of numerical simulations. Also, to verify the reliability of the prediction results from numerical simulations, the probability integral method was used for this prediction. The results show that the prediction results of the numerical simulations are close to those calculated by the probabilistic integral method, which proves the reliability of the numerical simulation prediction. The optimal mining scheme was determined by comparing the predicted results from the analysis of theoretical calculations and numerical simulations. [ABSTRACT FROM AUTHOR]

A reliable prediction of the strata behavior and support capacity of longwall workings has always been a challenge for mining engineers. It is still difficult to quantitatively predict some important mining parameters, such as support capability and the interval of periodic caving of the major roof under given mining and strata conditions. The reason is that several factors controlling the strata behavior are difficult to determine, such as the bulk factor of the caving zone, strength factor of the major roof and load factor of the overlying rock strata. In this study, the back analysis method is employed to address this problem. This paper illustrates an integrated approach to predict the strata movement behavior and optimum capability of powered roof supports for safe mining. First, this approach uses the empirical method to qualitatively analyze the mining influence zone, then establishes a quantitative prediction model by theoretic analysis method, and solves the optimum values of the factors described in the model with the back analysis method. On the basis of the above research, the support capability and interval of periodic caving of the major roof under similar geological conditions may be calculated by using these suitable values of the factors. Finally, an illustration of the above process is presented in this paper to examine the reliability of this approach. [ABSTRACT FROM AUTHOR]

In low-wall stability analysis, there are many geological factors that must be considered; therefore, in determining the method and parameters, real conditions in field must be taken into consideration. This paper examines what factors need to be considered in low-wall stability analysis, including condition of the study area. The analysis method used in this study was back analysis on a low-wall slope where failure occurred, by collecting all the details of geological conditions and performing finite-element-based stability analysis to break down the contribution of each factor. This method is expected to provide detailed information about geological condition that may be a contributing factor to slope stability analysis. The result showed that physical and mechanical properties, slope length, bed thickness, bedding ratio, lithology type, and aquifer type need to be considered. These considered conditions become important factors in the analysis, especially in determining suitable slope stability method as well as evaluating reasonable results of low-wall stability analysis. Comprehensive slope stability analysis may help improving quality of slope to be optimal. The result of this analysis can be implemented for layered sedimentary rocks with low mechanical properties. [ABSTRACT FROM AUTHOR]

Rock burst of soft coal is a new strongly concealed dynamic hazard, which threatens the mining safety of deep mine due to its unknown occurrence mechanism and the difficulty in prevention and control. Taking a coal mine in a rich coal zone with hidden hazards as an example, this paper firstly studied the cause of burst hazard in the region of deformed soft coal through theoretical analysis and field measurement, analyzed the evolution of soft coal burst, revealed its mechanism and finally proposed specific prevention and control measures. The main conclusions are: (1) the primary cause of impact hazard in soft coal mines was the formation of extensive disturbance zone under high stress, which caused the dirt ban to form a structure for carrying stored energy after the stages of dispersion, suspension, sliding motion and embedded squeeze contact; (2) the evolution of the rock bust of soft coal was concealed. In the process of the formation of bearing and storage structure, external input load gradually dissipated due to the severe deformation of roadway, forming an illusion without hazard; (3) the burst of soft coal was essentially the unstable impact of dirt band, so the criteria of obtaining stress and power for structural unstable impact was reduced; (4) the measures against the burst of soft coal from identifying potential impact areas, synchronously monitoring the stress of coal and the deformation of roadway to implementing prevention measures, were proposed, which exhibited a positive effect. [ABSTRACT FROM AUTHOR]

Deep coal mining changes the original rock state, stress concentration occurs in the process of self-stabilizing adjustment of surrounding rock. At this time, rock burst is very easy to occur, which brings great threat to mine safety production. Based on catastrophe theory, the energy catastrophe criterion, displacement mode catastrophe criterion and entropy catastrophe criterion for surrounding rock system of deep buried tunnel are established in this paper. And the criteria are applied to the stability judgment of surrounding rock in Binchang coal mine. The analysis results show that the energy eigenvalue, displacement eigenvalue and entropy information eigenvalue for surrounding rock of deep buried tunnel suddenly jump and the surrounding rock loses stability when the excavation reaches the fourth step. The accuracy of the criterion is verified by laboratory test and FLAC3D numerical simulation. The above criteria can be used to judge the possibility of instability from different aspects according to the system mutation conditions, which can provide some reference for preventing local surrounding rock system instability in the process of excavation disturbance of deep buried tunnel. [ABSTRACT FROM AUTHOR]

During the mining process of short distance coal seams group with high gas content, excavation of the protective coal seam will result in stress relief and fracture development which provides pathways for gas migration. Gas stored in the lower coal seam moves along the mining-induced fracture to the floor of the upper working face, which may cause the gas over-limit at the upper coal seam. This paper takes the short distance coal seams group in Xinghua coal mine as an example, theoretical analysis, numerical simulation and field monitoring are comprehensively applied to study the floor failure mechanism, pressure relief law and dynamic evolution of cracks induced by the mining of the second-right working face. The results showed that as the upper coal seam gradually advances, the pressure relief angle of roof and floor gradually decreases, and the pressure relief range and floor failure depth gradually increase. After the working face of the upper coal seam is advanced 100 m, the range of the pressure relief in the surrounding coal and rock masses approaches stable; the cracks in the shallow depth are mainly with small angles or type II shear cracks, and the cracks in the large depth are mainly shear-expansion with lager angles, and the amount of fractures in the floor reduces with the increase of the depth of the floor. It was found that the mining-induced fractures in the floor is distributed in the shape of an "O" ring, and as the working face advances, the range of this "O"-shaped crack circle keeps moving forward but the width of the crack circle is basically unchanged. Finally, the low-level drilling field was used to control the gas from lower coal seams according to the stress relief and fracture distribution law, which achieved satisfied application effect and provided guidance for mine gas extraction in similar mining conditions. [ABSTRACT FROM AUTHOR]

This paper is a case study on the key technique of stability control of surrounding rock in the deep chamber with large cross-section in Zhangji Coal Mine of China. Many investigations and field experiments were performed to reveal the surrounding rock failure mechanisms of the chamber. It was found that the large cross-section, high in situ stresses, large rock zone and construction disturbance were the main causes of the chamber instabilities. For the original supporting scheme, the numerical simulation was adopted to analyze the surrounding rock stress and displacement during excavation and support. There are stress concentrations and large displacement at the junction of the chamber, shaft lining, and machine roadway. The maximum displacements are 74.7 mm in horizontal, 105.9 mm roof fall and 29.2 mm floor heave in vertical. Based on the simulation results, the supporting structure optimization design of the chamber has been completed. The primary supporting structure is composed of the bolt, steel mesh, shotcrete and anchor. The secondary supporting is concrete lining with double-layer reinforcement and an inverted arch. The shallow and deep lagging grouting technique was adopted for reinforcing rock. The three-dimensional steel grid inverted arch lining was developed independently. The indoor model test showed that the inverted arch lining can take advantages and characteristics of various materials through unique spatial network design, which improved the whole bearing capacity. A field experiment of convergence and reinforcement stress monitoring was conducted. The field measurement data showed that the maximum displacement of two sides and roof were 60 mm and 42 mm at 200 days, respectively. The reinforcement stresses of supporting structure were far less than the yield strength, which had a higher safety stock. This successful case study demonstrated that the optimized supporting structure is reasonable and the deformation of surrounding rock has been effectively controlled. [ABSTRACT FROM AUTHOR]

This paper adopts an integrated method of field test and numerical simulation to study coal pillar width and surrounding rock control of gob-side entry in extra-thick coal seam. Field test showed that when the coal pillar width was 30 m, the average plastic failure areas of the roof, coal pillar rib and virgin coal rib were 7.6 m, 4.9 and 3.8 m, respectively, and severe roof subsidence and large deformation of coal pillar rib occurred during the mining of the working face. A numerical model is established to study the change of plastic bearing zone and the distribution of stress and displacement under different pillar widths. The numerical results show that a coal pillar 8 m wide can meet the strength requirement of overlying load and puts the gob-side entry in a good stress environment. Finally, an optimal coal pillar width was estimated at 8 m and an effective support method was proposed. Drilling imaging data indicate that the newly designed pillar size and the proposed supporting method can ensure roadway stability. The proposed plastic bearing zone and zero-displacement plane are reliable indicators for designing pillar size. In addition, the new support strategy can be applied to other coal mines with similar geological conditions. [ABSTRACT FROM AUTHOR]

The influence of dirt band on the top coal caving property is usually analyzed from the thickness and hardness of the dirt band. For the extra-thick coal seam with a thickness of more than 20 m, the location of the dirt band also has a huge impact on the top coal caving property. In this paper, No. 19 coal seam of Tanyaoyu coal mine is took as the research object, the influence of the location of dirt band on the top coal caving property is analyzed, and the calculation formula of the limit span and thickness of the dirt band is derived. The results show that, when the dirt band located in the upper part of the extra-thick coal seam, the bridge structure is more to form in the mining process, when the dirt band located in the middle part of the extra-thick coal seam, the arch structure is more to form, when the dirt band located in the lower part of the extra-thick coal seam, the dirt band is to release in the form of a loose media. The upper dirt band and lower dirt band have little influence on the top coal caving property, while the middle part dirt band has the greatest influence on the top coal caving property. [ABSTRACT FROM AUTHOR]

The overlying strata in mining could move, deform and break with the advancement of the underground mining operations and form the "three-vertical-zones" over the mined-out areas. In this paper, the influence function method based to predict the surface subsidence is innovatively introduced to establish the movement model of subsurface strata over underground mine gob. In order to validate the developed model's reliability, the model is applied in a Chinese underground mine to calculate the subsurface strata movements. Then a physical simulation modelling is also conducted to study the behavior of rock strata affected by mining. By using cross-validation, the theoretical results are compared with the physical simulations, which indicates that those two results are highly agreed with each other. The reliability of developed model can be a good tool to study the progress of subsurface stratum bedding and caving in a coal mine. [ABSTRACT FROM AUTHOR]

In this paper, a roof falling and failure mechanism of roadway with water-rich surrounding rock is deduced based on upper bound method of plastic mechanics and variational principle. For the derivation, a roof falling structure of the roadway is constructed according to the characteristics of the roadway; and it is also based on Hoek-Brown strength criterion and its associated flow rules. The structure is made with the consideration of the influence from stress level on roof rock, formation water pressure and supporting load. A typical roadway with water-rich surrounding rock is taken as an example of field practice to analyze the influence of different mechanical parameters on roof failure mechanism. The roadway is located in Guotun Coal Mine, which is over a 1000 m deep. The calculation results of this study show the size of falling piece from roof represents the safety performance of roof; the size will be enlarged along with the increase of empirical parameter A, tensile strength, compressive strength and supporting load of rock; and meanwhile the size will be reduced along with the increase of empirical parameter B, stress and weight of rock. Among all the factors, tensile strength of rock and stress level of roof rock have a significant effect on roof failure mechanism; parameter B determines the fracture shape of the falling piece from roof; and the fracture curvature of the falling piece decreases with the increase of the parameter B. [ABSTRACT FROM AUTHOR]

Today one of the world's largest coal mining producers is China. Mining, especially coalmines had been the major sources of income to Chinese people. The mining activity offers many great opportunities to country and its population. Unlike in Ivory Coast where the active involvement of many people in artisanal mining activities in Ivory Coast is mainly due to the high rate of abject poverty and the low price of agricultural produce. With the increasing depth of mining, rock burst is on the rise. China's deep mining industry faces such a threat. A major problem of deep mining is high ground stress, which is the major factor of rock burst. Due to the complexity of the rock burst mechanisms, complexity of induced factors as well as suddenness and randomness of rock burst occurrences, studies of rock burst prediction and control for safe mine exploitation is far from satisfying. The purpose of this paper is to examine of rock burst phenomenon in order to understand both stability in rock mechanics and the prediction of instability disasters in rock mechanics for application in the Ivory Coast mining sector with reference to the current Chinese mining industry. [ABSTRACT FROM AUTHOR]

Dragline is highly capital intensive equipment to procure, operate and maintain in any surface mining operation. Given this, every second of operation of this capital intensive equipment is absolutely important. Improvement of even a single second in the total cycle time has a tremendous bearing on the overall performance of this equipment. In this light, the present paper is an endeavour to critically analyze the cycle time of dragline operations in a major surface coal mine in India. Rigorous statistical analysis has been performed on individual cycle time segments, of complete dragline cycle. The segmental cycle times have been found to be statistically significant and appear to be best represented by lognormal, normal and beta distributions. Furthermore, the mean time of the statistical distribution for segmental cycle time of dragline has revealed the dependence of cycle time on cut geometry and depth. Results have been illustrated in the form of figures, graphs and tables. [ABSTRACT FROM AUTHOR]

Ground penetrating radar (GPR) has been proven to be one of the most promising non-destructive methods that has been widely used in engineering quality inspections and condition assessments of underground structures. This paper reports an application study of GPR that presents a comprehensive survey of the auxiliary shaft of the Anli coal mine in China. During shaft installation, water gushing with a maximum inflow rate of approximately 16.0 m3/h occurred, and thus, it was urgent to determine the distribution range of anomalies and evaluate the lining thickness of the shaft to provide precise technical guidance for later treatments. A GPR grid scan was carried out by towing the antenna along 6 vertical measuring lines and 11 orthogonal circular measuring lines on the shaft surface. The GPR results show that the anomalies experienced by the shaft are voids, uncompacted areas and water bodies. Most anomalies were located in the outer shaft lining and surrounding soil. A vast majority of the shaft lining thickness reach its designed value, with a qualified rate of 91.2% for the inner shaft lining and 89.9% for the outer shaft lining, and the outer shaft lining exhibited inferior smoothness and uniformity. Under the guidance of the GPR results, a grouting treatment was carried out to water seal and strengthen the shaft lining, which, consequently, reduced the water inflow rate by 90%. The favourable grouting effect demonstrates that the GPR method can feasibly and successfully reflect the shaft lining and anomalies in shaft assessments. This engineering practice could serve as a case study reference for investigating any similar underground structure in the future. [ABSTRACT FROM AUTHOR]

With the persistent growth in the global population and the rapid development of industrial engineering, the significance to the safety management of engineering practice has become increasingly prominent. The evaluation method for safety performance is a complex systematic problem in the safety management of industrial engineering. This paper is to introduce a safety evaluation method referring to the integrated treatment of safety management. It selected accident loss and accident rate as evaluation indexes, and identified a reasonable index system. Based on this risk index system, the mathematical model of the differential equation using grey variable weight clustering analysis is set up to evaluate the safety performance. The validity of the method is verified by the numerical example of a coal mine in China. According to the results of clustering, the safety performance is divided into three grades of "better", "moderate" and "inferior". Moreover, the future safety performance is predicted further by using GM (1,1) dynamic model, and the relevant prevention measures are put forward. A set of systematic method of safety performance evaluation (SPE) and risk prediction is presented for industrial engineering and practice, and provide a reference for safety management. [ABSTRACT FROM AUTHOR]

In order to prevent water inrush from working face floor, it is an urgent problem to predict destroyed floor depth. In this paper, based on the collected samples of floor failure depth of north China type coal fields, the factors that influence the development of floor failure depth were obtained by principal component analysis(PCA), that is, mine pressure, working face information, tectonic conditions, lithology, and sedimentary conditions of coal seam. Then, the parameters of support vector regression (SVR) were optimized by genetic algorithm (GA), the PCA-GA-SVR prediction model of floor failure depth was established, we put the last 4 samples into the model and verified its excellent generalization ability. Finally, we applied the PCA-GA-SVR model to the floor failure depth prediction of Zhaizhen coal mine, comparing the predicted value of the model with the field measured value using double side seal borehole water injection device, the effectiveness of the model was demonstrated. [ABSTRACT FROM AUTHOR]

Bending strata is an inevitable consequence of coal mining, which will waste mineral resources and influence their production. In this paper, the displacement function suitable for curved beam in polar coordinates is introduced, and the partial differential governing equation of curved beam is obtained by theoretical analysis. Then, the expressions of displacement components and stress components are expressed by displacement function. On this basis, the program for solving the partial differential equation is compiled using the difference principle. Finally, these theoretical formulas of curved beam are applied to analyze the displacement distribution of bending rock and the influencing factors of deformation in coal mining. These main results show that the value of deformation is increasing with the increase of inner radius, tectonic stress and mining depth, while it is decreasing with the increase of thickness of overburden layer. Moreover, the advancing angle effects significantly not only the value of deformation, but also the position of max value. The value of deformation is more suitable for mining position at about 30°. Obviously, the inner radius and advancing angle have more effect on deformation than other factors. These conclusions provide scientific basis and reference for coal mine engineering. [ABSTRACT FROM AUTHOR]

The mining subsidence basins are mostly irregular. The traditional method is difficult to achieve quantitative evaluation of mining subsidence damage. This paper investigated the quantitative indicators of land subsidence damage evaluation based on GIS. The calculation method of the area and volume of surface subsidence caused by underground mining was proposed. Firstly, the principle, requirements, methods and steps of drawing the sinking boundary line were given. A calculation method based on the subsidence area of the surface subsidence database was proposed. According to the sinking contour map, the calculation method of subsidence area based on the sinking contour map was proposed. Based on the calculation method of surface subsidence area and and the digital elevation model, the calculation method of the mining subsidence volume was put forward. The surface subsidence area and volume were calculated by numerical examples. This realized the quantitative evaluation of surface subsidence damage caused by underground coal mining, and promoted the application of GIS in mining damage assessment. [ABSTRACT FROM AUTHOR]

Coal is China's dominant energy at present,but gas outburst occurs frequently in gassy coal mines, and it causes significant losses to people's lives and properties. Hydraulic fracturing is a valuable technology for gas permeability improvement in gassy coal mines. In this paper, the field experiment of hydraulic fracturing was conducted in a coal mine located in Yibin City, Sichuan Province. The transient electromagnetic instrument was applied before and after hydraulic fracturing respectively, combined with numerical simulation analysis of FLAC3D, aimed to estimate the influence scope of hydraulic fracturing. The numerical simulation results denote that the plastic zone principally distributes along the interface of the coal seam and roof due to the existence of stratification. Transient electromagnetic method analysis demonstrates that the coal-rock mass firstly cracks at 35 m and 41 m of borehole depth, the dominant fissure distribution range is in the interval of 33–50 m. And the influence area covers the range of 28–64 m, which means the influence scope is up to 36 m. It also reveals that it is feasible to determine the influence scope of hydraulic fracturing through transient electromagnetic method. [ABSTRACT FROM AUTHOR]

Various dynamic disasters will happen in the extremely soft coal seam mining process of Gob-side entry in the deep mine, including large deformation of roadways, rock burst, gas unusual, floor water invasion, spontaneous combustion of coal seam and so on. Therefore, a reasonable segment pillar will not only avoid the dynamic disasters in the excavating process, but also improve the resource utilization. Taking the goaf working face of extremely soft coal seam in the deep mine in one particular mine of Shandong Province as the engineering example, this paper first analyzes optional locations of the gob-side entry, finding that the gob-side entry should be located in the low stress block and it's the most favorable to keep small pillars and goafs isolated. Then, it is found that small surrounding rock deformation of gob-side entry takes place in the driving process through the numerical simulation, and the deformation will increase with the increasing width of the chain pillar. The surrounding rock deformation of gob-side entry increases in the stoping stage. Small width of the chain pillar will easily result in the instability of the roadway. As the swivel angle of the basic roof increases, the surrounding rock deformation of gob-side entry will accordingly increase. Finally, the numerical calculation and on-spot mine pressure monitoring illustrate that the width of the segment pillar in the gob-side entry should be 5–7 m, with 6 m the best. [ABSTRACT FROM AUTHOR]

The stress distribution of surrounding rock is uneven under mining dynamical pressure, which results in serious breakage. And roadway sides slide inward that leads to roof further crushed. In this paper, the theory of self-stabilizing balanced arch is used to analyze the failure mechanism of surrounding rock and to calculate the range of easy falling area. That can effectively prevent breakage on roadway and can increase strength supporting reasonably. According the supporting situation on the Guotun coal mine, author proposed two feasible supporting schemes. By comparing FLAC3D simulation deformation maps, we can see that increasing appropriately the density of bolt in the roadway sides that can increases the strength of surrounding rock. That also can increases the supporting ability to roof. With the Guotun coal mine as the engineering background, author finally elected the strengthen asymmetric supporting scheme. That increases the supporting strength at the roadway sides and corners. And the concrete scheme is that the different numbers and the same type Φ20 × 2400 mm (Q500) bolts respectively arranged on the goaf side and the solid coal. Every of the roadway corners arranged a Φ21.6 × 4000 mm anchor cable with an inclination of 19°. Roadway roof arranged three the same type anchor cable. They can make "top-side-bottom" combination anchored in the stable rock stratum. The engineering observation showed that the max roadway roof deformation was 42 mm on the 26th day and that of two roadway sides were 30 mm on the 24th day and there remained stable afterwards. Asymmetric supporting in roadway sides that can balances the stress of surrounding rock and can reduces roadway sides slide inward. Increasing supporting strength to roadway sides and corners that can makes the "top-side-bottom" combination be more and more stable. This scheme can better solves the uneven deformation problem on roadways and guarantees faster and safer production. [ABSTRACT FROM AUTHOR]

In situ measurements of deformations, stresses, and closure of fractures, affecting water inflow following coal mining, are challenging due to the inaccessibility of fractured rock. In this paper, the authors studied the closure process of the fractured rock mass with the cover stress re-establishment based on a theoretical analysis and a scale model testing. A quantitative analysis is used to study the fracture distribution in the fractured zone. A function to describe a fracture aperture distribution in the fractured zone is proposed, which takes into account the curvature and thickness of the fractured rock. The theoretical analysis and a scale model testing both indicate that the cover stress re-establishment with mining distance increasing and the relationship between the fracture closure and cover stress re-establishment both satisfy a logarithmic function. The scale model test also shows the following features: (1) the fracture ratio (which is the fracture area divided by the total area of fracture and intact rock with a unit width in the vertical or horizontal direction) in the lower part of the fractured rock mass is greater than that in the upper part; (2) the initially fast decreased of fracture ratios is then followed by a slower decrease during the cover stress re-establishment process; (3) in the upper part of the rock mass, the vertical directional fractures with small apertures are being closed with cover stress re-establishment, which indicates an increase in the water resistance reducing the seepage from these parts of the fractured zone. This study improves the general understanding of the fracture closure process and cover stress re-establishment in the fractured rock mass after coal mining ceased, and provides a theoretical basis for water resource protection in case of underground coal mining. [ABSTRACT FROM AUTHOR]

How to describe the compression creep behavior of gangue is very essential for the design of gangue backfilling mining. In this paper, compression tests of two kinds of gangue are studied, finding that logarithmic function is suitable for describing its equal-time stress-strain relationship. Stress-strain-time relationship is proposed through modifying the Singh-Mitchell creep model based on the analysis of the test result, and this model can provide a design basis for gangue backfilling technology. Back analysis indicates that this model can basically describe the creep behavior of gangue with a few parameters that are easy to be determined. This research can provide a reference on the design of gangue backfilling mining, and on the subsidence value estimation. [ABSTRACT FROM AUTHOR]